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United States Patent |
6,071,783
|
Liang
,   et al.
|
June 6, 2000
|
Pseudo silicon on insulator MOSFET device
Abstract
A process for forming a MOSFET device, featuring a heavily doped
source/drain region, isolated from a semiconductor substrate, via use of a
thin silicon oxide layer, has been developed. After formation of a lightly
doped source/drain region, an opening is created in the semiconductor
substrate, in a region between insulator spacers, on a gate structure, and
insulator filled, shallow trench regions, resulting in lightly doped
source/drain segments, remaining under the masking insulator spacers.
After a thin silicon oxide layer is formed on the exposed silicon
surfaces, in the openings, a silicon deposition, and etch back procedures
are performed, partially refilling the openings to a depth that still
allows the thin silicon oxide layer to be exposed on the sides of the
lightly doped source/drain segment. After removal of the exposed portion
of the thin silicon oxide layer, and after deposition and etch back of
another silicon layer, completely filling the openings, a heavily doped
source/drain region is formed in the silicon layers, residing in the
openings.
Inventors:
|
Liang; Mong-Song (Hsin-chu, TW);
Lee; Jin-Yuan (Hsin-chu, TW);
Yoo; Chue-San (Hsin-chu, TW)
|
Assignee:
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Taiwan Semiconductor Manufacturing Company (Hsin-Chu, TW)
|
Appl. No.:
|
133353 |
Filed:
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August 13, 1998 |
Current U.S. Class: |
438/301; 257/E21.431; 257/E29.021; 257/E29.121; 438/264; 438/300; 438/305 |
Intern'l Class: |
H01L 021/336 |
Field of Search: |
438/300,305,264,424,291,301,282,289,302
|
References Cited
U.S. Patent Documents
5296392 | Mar., 1994 | Grula et al. | 437/34.
|
5575885 | Nov., 1996 | Hirabayashi et al. | 437/228.
|
5610083 | Mar., 1997 | Chan et al. | 437/21.
|
5691231 | Nov., 1997 | Kobayashi et al. | 437/62.
|
5786229 | Jul., 1998 | Park | 437/67.
|
5872043 | Feb., 1999 | Chen | 438/424.
|
5943575 | Aug., 1999 | Chung | 438/300.
|
Primary Examiner: Niebling; John F.
Assistant Examiner: Lindsay, Jr.; Walter L.
Attorney, Agent or Firm: Saile; George O., Ackerman; Stephen B.
Claims
What is claimed is:
1. A method of fabricating a MOSFET device, on a semiconductor substrate,
comprising the steps of:
forming isolation regions in said semiconductor substrate;
forming a gate insulator layer on said semiconductor substrate;
forming an insulator capped, gate structure on said gate insulator layer;
forming a lightly doped source/drain region, in regions of said
semiconductor substrate, not covered by said insulator capped, gate
structure, and not covered by said isolation regions;
forming an insulator spacer on the sides of said insulator capped, gate
structure;
forming first openings in a region of said semiconductor substrate, between
insulator spacers, on the side of said insulator capped, gate structure,
and said isolation region, removing the portion of said lightly doped
source and drain region, exposed between insulator spacers, on the sides
of said insulator capped, gate structure, and isolation regions, and
leaving lightly doped source/drain segments, under said insulator spacers;
growing a thin insulator layer on the exposed surfaces of said first
openings, including the growth of said thin insulator layer on the exposed
side of said lightly doped source/drain segments;
partial filling of said first openings, with a first silicon layer, on said
thin insulator layer, in said first openings, creating second openings,
located between said insulator spacers, on the sides of said gate
structure, and said isolation regions, with said second openings exposing
a portion of said thin insulator layer, located on the side of said
lightly doped source/drain segments;
removing said portion of said thin insulator layer, from the side of said
lightly doped source/drain segments, in said second openings;
complete filling of said second openings, with a second silicon layer, and
with said second silicon layer interfacing the side of said lightly doped
source/drain segments, in said second opening; and
forming heavily doped source/drain regions in said second silicon layer,
and in said first silicon layer, with said heavily doped source/drain
regions, interfacing the side of said lightly doped source/drain segments,
in a region underlying said insulator spacers, and with said heavily doped
source/drain region interfacing, and overlying, said thin insulator layer.
2. The method of claim 1, wherein said isolation regions, are insulator
filled shallow trench regions, formed to a depth between about 2000 to
5000 Angstroms, in said semiconductor substrate, and filled with silicon
oxide.
3. The method of claim 1, wherein said gate insulator layer is silicon
dioxide, obtained via thermal oxidation, in an oxygen-steam ambient, at a
thickness between about 25 to 100 Angstroms.
4. The method of claim 1, wherein said insulator capped, gate structure is
a polysilicon gate structure, formed from a polysilicon layer, obtained
via LPCVD procedures, at a thickness between about 1000 to 3000 Angstroms.
5. The method of claim 1, wherein the insulator layer, used for said
insulator capped, gate structure, is a silicon oxide layer, obtained via
LPCVD or PECVD procedures, at a thickness between about 200 to 2500
Angstroms.
6. The method of claim 1, wherein the insulator layer, used for said
insulator capped, gate structure, is a silicon nitride layer, obtained via
LPCVD or PECVD procedures, at a thickness between about 200 to 2500
Angstroms.
7. The method of claim 1, wherein said lightly doped source/drain region is
created via ion implantation of arsenic or phosphorous ions, at a energy
between about 5 to 30 KeV, at a dose between about 1E13 to 5E14
atoms/cm.sup.2.
8. The method of claim 1, wherein said insulator spacers are formed from a
silicon oxide layer, obtained using LPCVD or PECVD procedures, at a
thickness between about 500 to 2000 Angstroms.
9. The method of claim 1, wherein said insulator spacers are formed from a
silicon nitride layer, obtained from a LPCVD or PECVD procedure, at a
thickness between about 500 to 2000 Angstroms.
10. The method of claim 1, wherein said first openings are formed to a
depth between about 1000 to 3000 Angstroms, below the top surface of said
semiconductor substrate, via an anisotropic RIE procedure, using Cl.sub.2
as an etchant.
11. The method of claim 1, wherein said thin insulator layer, on the
surface of said first openings, is a silicon dioxide layer, obtained via
thermal oxidation procedures, in an oxygen-steam ambient, to a thickness
between about 40 to 50 Angstroms.
12. The method of claim 1, wherein said first silicon layer, used to
partially fill said first openings, is a polysilicon layer, or an
amorphous silicon layer, obtained via LPCVD procedures, to a thickness
between about 1000 to 6000 Angstroms.
13. The method of claim 1, wherein said partial filling of said first
openings, is accomplished via deposition of said first silicon layer, and
an anisotropic RIE, etch back procedure, using Cl.sub.2 as an etchant for
said first silicon layer, creating said second opening, with a depth
between about 300 to 1500 Angstroms, below the top surface of said
semiconductor substrate.
14. The method of claim 1, wherein the portion of said thin insulator
layer, exposed in said second opening, located on the side of said lightly
doped source/drain segment, is removed using a buffered hydrofluoric acid,
or a dilute hydrofluoric acid, solution.
15. The method of claim 1, wherein said second silicon layer, used to fill
said second openings, is a polysilicon layer, or an amorphous silicon
layer, obtained via LPCVD procedures, at a thickness between about 1000 to
6000 Angstroms.
16. The method of claim 1, wherein said heavily doped source/drain region
is created via ion implantation of arsenic or phosphorous ions, at an
energy between about 10 to 50 KeV, at a dose between about 1E15 to 1E16
atoms/cm.sup.2.
17. A method of fabricating a MOSFET device, on a semiconductor substrate,
using a thin silicon oxide layer to isolate a heavily doped source/drain
region, from said semiconductor substrate, comprising the steps of:
forming isolation regions in said semiconductor substrate;
forming a silicon dioxide gate insulator layer on said semiconductor
substrate;
forming a silicon oxide capped, polysilicon gate structure, on said silicon
dioxide gate insulator layer;
forming a lightly doped source/drain region, in a region of said
semiconductor substrate, not covered by said silicon oxide capped,
polysilicon gate structure, or by said isolation regions;
forming silicon oxide spacers on the sides of said silicon oxide capped,
polysilicon gate structure;
anisotropic etching of a region of said lightly doped source/drain region,
and of an underlying portion of said semiconductor substrate, exposed
between said silicon oxide spacers, and said isolation region, to create
first openings in said semiconductor substrate, leaving lightly doped
source/drain segments under said silicon oxide spacers;
growing said thin silicon oxide layer on the exposed surfaces of said first
opening;
depositing a first intrinsic silicon layer, completely filling said first
openings;
etching of said first intrinsic silicon layer, to a level in which first
silicon layer remains in the bottom portion of said first openings,
resulting in second openings, located between said silicon oxide spacers,
and said isolation regions, with said thin silicon oxide layer, on sides
of said lightly doped source/drain segments, exposed in said second
openings;
removing said thin silicon oxide layer, from the sides of said lightly
doped source/drain segments, exposed in said second openings;
depositing a second intrinsic silicon layer, completely filling said second
opening;
etching of said second intrinsic silicon layer, to a level in which said
second silicon layer is planar with the top surface of said semiconductor
substrate; and
forming said heavily doped source/drain regions, in said second intrinsic
silicon layer, and in said first intrinsic silicon layer, with said
heavily doped source/drain regions overlying said thin silicon oxide
layer, in said first openings, and with said heavily doped source/drain
regions, interfacing the sides of said lightly doped source/drain
segments, in regions of said second openings in which said thin silicon
oxide layer has been removed.
18. The method of claim 17, wherein said isolation regions are silicon
oxide filled, shallow trench regions, formed to a depth between about 2000
to 5000 Angstroms, below the top surface of said semiconductor substrate.
19. The method of claim 17, wherein said silicon dioxide gate insulator
layer is thermally grown, in an oxygen-steam ambient, to a thickness
between about 25 to 100 Angstroms.
20. The method of claim 17, wherein said silicon oxide spacers, on the
sides of said silicon oxide capped, polysilicon gate structure, are formed
via deposition of a silicon oxide layer, via LPCVD or PECVD procedures, at
a thickness between about 500 to 2000 Angstroms, followed by an
anisotropic RIE procedure, using CHF.sub.3 as an etchant.
21. The method of claim 17, wherein said lightly doped source/drain regions
are formed via ion implantation of arsenic or phosphorous ions, at an
energy between about 5 to 30 KeV, at a dose between about 1E13 to 5E14
atoms/cm.sup.2.
22. The method of claim 17, wherein said first openings are created via an
anisotropic RIE procedure, using Cl.sub.2 as an etchant, to a depth
between about 1000 to 3000 Angstroms, below the top surface of said
semiconductor substrate.
23. The method of claim 17, wherein said thin silicon oxide layer is
thermally grown on the exposed surfaces of said first openings, in a
oxygen-steam ambient, to a thickness between about 40 to 500 Angstroms.
24. The method of claim 17, wherein said first intrinsic silicon layer, is
an intrinsic polysilicon layer, or an intrinsic amorphous silicon layer,
deposited using LPCVD procedures, at a thickness between about 1000 to
6000 Angstroms.
25. The method of claim 17, wherein said thin silicon oxide layer, on the
sides of said lightly doped source/drain segment, exposed in said second
openings, is removed using a buffered hydrofluoric acid, or a dilute
hydrofluoric acid solution.
26. The method of claim 17, wherein said second intrinsic silicon layer is
an intrinsic polysilicon layer, or an intrinsic amorphous silicon layer,
deposited using LPCVD procedures, to a thickness between about 1000 to
6000 Angstroms.
27. The method of claim 17, wherein said heavily doped source/drain regions
are formed in said second intrinsic silicon layer, and in said first
intrinsic silicon layer, via ion implantation of arsenic, or phosphorous
ions, at an energy between about 10 to 50 KeV, at a dose between about
1E15 to 1E16 atoms/cm.sup.2.
Description
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to methods used to fabricate semiconductor
devices, and more specifically to a method used to fabricate a metal oxide
semiconductor field effect transistor, (MOSFET), device, in which the
source/drain region of the MOSFET device, is isolated from the
semiconductor substrate, by a thin silicon oxide layer.
(2) Description of the Prior Art
The semiconductor industry is continually striving to improve the
performance of devices, via use of micro-miniaturization, or the use of
sub-micron features. The objective of micro-miniaturization has been
successfully addressed via advances in specific semiconductor fabrication
disciplines, such as photolithography and dry etching. The use of more
sophisticated exposure cameras, as well as the use more sensitive
photoresist masking materials, have allowed sub-micron images to be
routinely achieved in masking photoresist layers. In addition the
development of advanced dry etching tools, and procedures, have allowed
the sub-micron images in photoresist masking layers, to be successfully
transferred to underlying materials, used for the fabrication of
sub-micron MOSFET devices. These sub-micron, MOSFET features, such as the
sub-micron channel length, located under narrow gate structures, between
source/drain regions, allow increased MOSFET performance to be achieved.
In addition the smaller MOSFET features, achieved via fabrication
enhancements, result in capacitance decreases, such as a decreased
source/drain to substrate capacitance, achieved via smaller source/drain
features, thus also resulting in improvement of MOSFET device performance.
In addition to MOSFET performance enhancements, achieved via
micro-miniaturization, and the accompanying reductions in performance
degrading capacitance, additional capacitance decreases, or performance
enhancements can be achieved via structural enhancements. The use of a
silicon on insulator, (SOI), technology, allowing the MOSFET device to be
fabricated on a SOI layer results in capacitance decreases between the
semiconductor substrate, and overlying, interfacing regions of the MOSFET
device. For example Chan et al. in U.S. Pat. No. 5,610,083, teach a
procedure for fabricating a MOSFET device on a SOI layer, and thus achieve
performance enhancements, when compared to counterparts, where the MOSFET
device was formed directly on a semiconductor substrate. However the use
of special semiconductor substrates, needed for the SOI technology, is
expensive, and although performance objectives may be satisfied using a
SOI wafer, the as important cost objectives, are not satisfied.
This invention will describe a procedure for obtaining a MOSFET in which an
insulator layer is formed only between heavily doped source/drain regions
and the semiconductor substrate. The use of this insulator layer, or the
pseudo SOI layer, results in capacitance reductions, performance
enhancements, via removal of the source/drain to substrate interface,
while maintaining cost, via use of a conventional semiconductor substrate.
SUMMARY OF THE INVENTION
It is an object of this invention to improve the performance of a MOSFET
device, by minimizing junction capacitance.
It is another object of this invention to fabricate a MOSFET device, using
a thin insulator layer only between heavily doped/source drain regions,
and the semiconductor substrate.
In accordance with the present invention a method for fabricating a MOSFET
device, using an insulator layer between a heavily doped source/drain
region, and the semiconductor substrate, used to decrease junction
capacitance, is described. A gate insulator layer is thermally grown on
regions of a semiconductor substrate, between isolation regions. A
polysilicon gate structure, with an overlying insulator hard mask, is
formed on the gate insulator layer. A lightly doped source/drain is formed
in the region of the semiconductor substrate, not covered by the
polysilicon gate structure, followed by the creation of insulator spacers,
on the sides of the polysilicon gate structure. An anisotropic, reactive
ion etching, (RIE), procedure is employed to remove regions of the
semiconductor substrate, not covered by the polysilicon gate structure,
not covered by the insulator spacers, and not covered by the isolation
regions, resulting in openings in the semiconductor substrate, between the
insulator spacers, on the polysilicon gate structure, and the isolation
regions, and also resulting in a segment of the lightly doped source and
drain region, underlying the insulator spacer. A thin insulator layer is
next thermally grown on the exposed surfaces of the opening in the
semiconductor substrate. A first silicon deposition, and etch back, is
performed to refill the opening in the semiconductor substrate, to a level
approximately equal to the bottom of the lightly doped source/drain
segment. The thin insulator layer, on the sides of the lightly doped
source/drain segment is next removed, followed by a second silicon
deposition, and etch back, completely filling the opening in the
semiconductor substrate. A heavily doped source/drain region is then
formed in the silicon fill, in the opening in the semiconductor substrate,
with the heavily doped source/drain region interfacing the region of the
lightly doped source/drain segment, located under the insulator spacer,
and with the reemainder of the heavily doped source/drain region residing
on the thin insulator layer, in the region in which the thin insulator
layer overlays the sides of the opening in the semiconductor substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
The object and other advantages of this invention are best described in the
preferred embodiment with reference to the attached drawings that include:
FIGS. 1-8, which schematically, in cross-sectional style, describe the key
stages of fabrication, used to create a MOSFET device, using a pseudo SOI
approach, or a thin insulator layer between source/drain and substrate,
for junction capacitance optimization.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
A method for fabricating a MOSFET device, using a thin insulator between a
heavily doped source/drain region, and the semiconductor substrate, will
now be described in detail. The MOSFET device used in this invention, is
an N channel, or NFET device, however this invention can be applied to P
channel, (PFET), devices, or to complimentary, (CMOS), device, comprised
of both NFET and PFET devices.
A P type, single crystalline, silicon substrate 1, with a <100>
crystallographic orientation, is used, and shown schematically in FIG. 1.
Shallow trench isolation, (STI), regions 2, are formed in the
semiconductor substrate, for purposes of isolation. Briefly, STI regions
2, are formed by initially creating a shallow trench, to a depth in
semiconductor substrate 1, between about 2000 to 5000 Angstroms, via
photolithographic and anisotropic RIE procedures, using Cl.sub.2 as an
etchant. After removal of the photoresist shape, used for definition of
the shallow trench, via plasma oxygen ashing and careful wet cleans, a
silicon oxide layer is deposited via low pressure chemical vapor
deposition, (LPCVD), or plasma enhanced chemical vapor deposition,
(PECVD), procedures, completely filling the shallow trench. Removal of
regions of the silicon oxide layer, overlying semiconductor substrate 1,
are removed using anisotropic RIE procedure, using CHF.sub.3 as an
etchant, resulting in STI regions 2, shown schematically in FIG. 1. A
chemical mechanical polishing, (CMP), procedure can be used in place of
the anisotropic RIE procedure, to remove unwanted regions of the silicon
oxide layer, also field oxide, (FOX), regions, created via thermal
oxidation procedures, can be used in place of STI regions, if desired.
A gate insulator layer 3, comprised of silicon dioxide, is next formed via
thermal oxidation in an oxygen-steam ambient, to a thickness between about
25 to 100 Angstroms. A polysilicon layer 4, is next deposited using LPCVD
procedures, to a thickness between about 1000 to 3000 Angstroms, using
silane as a source. Polysilicon layer 4, can be doped in situ, during
deposition, via the addition of arsine, or phosphine, to the silane
ambient, or polysilicon layer 4, can be deposited intrinsically, then
doped via ion implantation procedures, using arsenic or phosphorous ions.
In addition, if lower resistance is needed, polysilicon layer 4, can be
replaced by a polycide, (metal silicide-polysilicon layer), via deposition
of a metal silicide layer, such as tungsten silicide, on an underlying
polysilicon layer. Finally insulator layer 5, comprised of either silicon
oxide, or silicon nitride, is deposited using either LPCVD, or PECVD
procedures, to a thickness between about 200 to 2500 Angstroms.
Conventional photolithographic and RIE, procedures, using CHF.sub.3 as an
etchant for silicon oxide layer 5, (CHF.sub.3 would also be used as an
etchant if insulator layer 5, is silicon nitride), and Cl.sub.2 as an
etchant for polysilicon layer 4, are used to produce polysilicon gate
structure 4, capped with insulator layer 5. Removal of the photoresist
shape, used for definition of polysilicon gate structure 4, schematically
shown in FIG. 1, is accomplished via plasma oxygen ashing and careful wet
cleans.
A lightly doped source/drain region 6a, is next created via ion
implantation of phosphorous or arsenic, at an energy between about 5 to 30
KeV, at a dose between about 1E13 to 5E14 atoms/cm.sup.2, in a region of
semiconductor substrate 1, not covered by polysilicon gate structure 4.
Next an insulator layer 7, comprised of silicon nitride, is deposited,
again via use of either LPCVD or PECVD procedures, to a thickness between
about 500 to 2000 Angstroms. Insulator layer 7, can be a silicon oxide
layer, if desired, also obtained using LPCVD or PECVD procedures. An
anisotropic RIE procedure, using CHF.sub.3 as an etchant for silicon
nitride, (also using CHF.sub.3 as an etchant for silicon oxide), is used
to create insulator spacers 7, on polysilicon gate structures 4. This is
schematically shown in FIG. 2. A segment of lightly doped source/drain
region 6a, now resides under insulator spacer 7.
FIG. 3, shows the result of removing the region of semiconductor substrate
1, located between insulator spacers 7, on the sides of polysilicon gate
structure 4, and STI regions 2, accomplished via a selective, anisotropic
RIE procedure, using CL.sub.2 as an etchant. The use of Cl.sub.2 as an
etchant, allows semiconductor substrate 1, to be removed, while exposed
insulator layer 5, insulator spacers 7, and STI regions 2, are not
attacked. This procedure, creating opening 8a, with a depth between about
1000 to 3000 Angstroms, below the top surface of semiconductor substrate
1, removes exposed regions of lightly doped source/drain region 6a,
resulting in lightly doped source/drain segment 6b, protected by insulator
spacers 7, during the creation of opening 8a. A silicon dioxide layer 9,
is next grown, to a thickness between about 40 to 500 Angstroms, via the
thermal oxidation of the exposed surfaces of opening 8, in an oxygen-steam
ambient. Silicon oxide layer 9, is formed on the surfaces of semiconductor
substrate 1, and the surface of lightly doped source/drain segment 6b,
exposed in opening 8a. The formation of silicon oxide layer 9, on the
exposed side of lightly doped source/drain segment 6b, resulted in the
edge of lightly doped source/drain segment 6b, to now be located under
insulator spacer 7. This is shown schematically in FIG. 4.
A first deposition of an intrinsic silicon layer 10, comprised of either
polysilicon or amorphous silicon, is next performed via LPCVD procedures,
to a thickness between about 1000 to 6000 Angstroms, completely filling
opening 8a. If desired, an amorphous silicon layer can be deposited, then
annealed, to convert the amorphous silicon layer to a single crystalline
silicon layer. A first chemical mechanical polishing, (CMP), procedure is
used to polish silicon layer 10, followed by a first, selective
anisotropic RIE procedure, using Cl.sub.2 as an etchant, is used to etch
back the silicon layer 10, to a depth in which the top surface of silicon
layer 10, is between about 300 to 1500 Angstroms below the top surface of
semiconductor substrate 1, exposing the portion of silicon oxide layer 9,
which resides on the side of lightly doped source/drain segment 6b, and
creating opening 8b. This is schematically shown in FIG. 5. A isotropic
wet etch, using a buffered hydrofluoric acid solution, or a dilute
hydrofluoric acid solution, is used to remove the exposed portion of
silicon oxide layer 9, exposed in opening 8b, resulting in the exposure of
the side of lightly doped source/drain segment 6b. This is schematically
shown in FIG. 6.
A second deposition of an intrinsic silicon layer 11, is performed using
LPCVD procedures, to a thickness between about 1000 to 6000 Angstroms,
completely filling opening 8b. Silicon layer 11, can either be a
polysilicon layer, a amorphous silicon layer, or a amorphous or
polysilicon layer, subjected to an anneal procedure to create a single
crystalline silicon layer. A second CMP procedure, followed by a second,
selective anisotropic RIE procedure, using Cl.sub.2 as an etchant, is used
to etch back silicon layer 11, to a depth in which the top surface of
silicon layer 11, is at the same level as the top surface semiconductor
substrate 1. In addition silicon layer 11, interfaces the side of lightly
doped source/drain segment 6b, in the region directly underlying insulator
spacer 7. This is schematically shown in FIG. 7. Finally a ion
implantation procedure is performed, using phosphorous or arsenic ions, at
an energy between about 10 to 50 KeV, at a dose between about 1E15 to 1E16
atoms/cm.sup.2, followed by an anneal procedure, at a temperature between
about 700 to 1100.degree. C., creating heavily doped source/drain region
12, in silicon layer 10, and silicon layer 11. Heavily doped source/drain
region 12, shown schematically in FIG. 8, is isolated from semiconductor
substrate 1, via silicon oxide layer 9, and thus offers less junction
capacitance, and increased performance, compared to counterparts
fabricated using heavily doped source/drain regions in direct contact with
the semiconductor substrate. The only region in which the heavily doped
source/drain region 12, is not isolated by silicon oxide layer 9, is where
contact to lightly doped source drain segment 6b, occurs. Thus the process
for creating a pseudo MOSFET device, using a thin silicon oxide layer, to
reduce junction capacitance arising at the interfaces of MOSFET regions,
can be used as an inexpensive alternative to creating MOSFET devices in a
SOI layer.
While this invention has been particularly shown and described with
reference to, the preferred embodiments thereof, it will be understood by
those skilled in the art that various changes in form and details may be
made without departing from the spirit and scope of this invention.
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